1. Field of the Invention
This invention relates generally to a method and an apparatus for optically recording digital signals on a disc, and more particularly to such a method and an apparatus for optically recording time-sequential signals such as an audio signal and a video signal on a disc-shaped recording medium.
2. Description of the Prior Art
There is known a hard disc drive apparatus which is provided with a disc-shaped recording medium and is capable of storing a great deal of data on the disc-shaped recording medium.
This hard disc drive apparatus is mainly used as a mass data storage device for computer processing. The disc-shaped recording medium arranged in the hard disc drive apparatus is driven at a predetermined rotational speed irrespective of the transmission rate of data to be recorded thereon. The data is magnetically recorded on cylindrical tracks formed on the disc.
There has also been developed a technique to optically record data on a disc-shaped recording medium, for example, a so-called WORM (write once read many) disc or MO (magneto-optical) disc. The WORM disc is exposed to a laser beam, the light intensity of which is changed in response to data to be recorded thereon so that pits are formed on the WORM disc to thereby record the data thereon. Data is recorded on the MO disc by applying a magnetic field having a predetermined magnitude to a magnetic film formed on the disc and at the same time exposing the film to a laser beam, whose light intensity is changed in response to the data to be recorded, to thereby control the magnetization direction of the magnetic film with the result that the data is recorded magneto-optically on the MO disc.
In the case of the WORM disc, the data once recorded thereon can not be erased while the data recorded on the MO disc can be erased. It is therefore possible in the case of the MO disc to record data again in the data recording region on which once recorded data has been erased.
A disc drive apparatus provided with a disc-shaped recording medium on which data is optically recorded can also be used as a mass data storage device for computer processing. Accordingly, the disc-shaped recording medium, on which the data is optically recorded, is rotated at a predetermined constant speed irrespective of the transmission rate of the data to be supplied and to be recorded to the disc drive apparatus.
There are a number of ways that time-sequential data can be generated and the transmission rate of such time-sequential data can differ from one source to another. Such time-sequential data can be, for example, an audio signal or video signal which is transmitted through a telephone line as digital data at a regulated standard transmission rate, e.g. as 1.536 Mbps in Japan and 2.048 Mbps in the European Community.
A compact disc (CD) player is an apparatus which reproduces an audio signal recorded on a compact disc as digital data. The so-called 8 mm video recorder (VTR) is an example of an apparatus which can transform an audio signal into a digital signal, record the transformed digital signal with a video signal on a 8 mm width magnetic tape, and reproduce the recorded signals therefrom. Further, there is a digital audio tape recorder using rotary heads (so-called R-DAT) which can record and reproduce an audio signal.
The sampling frequency of the audio signal for the CD player is regulated as 44.1 kHz, and the sampling frequency for reproducing the audio signal in the 8 mm VTR is about 31.5 kHz. In the case of the R-DAT, it is possible to select as an adequate sampling frequency from 32 kHz, 44.1 kHz, or 48 kHz and then an audio signal is reproduced. Furthermore, when an audio signal is sampled and then recorded, there is a variation in the quantitization number or sampled data length such as 8 bits, 12 bits, 16 bits, and so on, dependent on the respective apparatus. As a result, the transmission rate of the data to be reproduced by the respective apparatus differs from one apparatus to another.
It was for these reasons heretofore impossible to record digital data having different transmission rates on one disc.